Noise from external sources can create additional measurement problems, but can be mitigated in much the same way as thermocouples — by using differential, ungrounded, and shielded elements. These effects can also be limited by optional electronics that perform 10%-duty cycle measurements to limit self-heating power without reducing signal strength. However, the trade-off for utilizing low-level signals (power) to drive an RTD is that measures may be required to minimize the effect of external noise.
When building your knowledge base on sensor types, be sure to consider inherent accuracy for durability, range of operation, and susceptibility to external noise influences. Other considerations include: sensor temperature limits compared to process requirements (limits should bracket process range), the required level of accuracy and repeatability, ease of maintenance and installation, handling during installation (delicacy), ease of calibration, and the type of environment it will be used in.
An awareness of the inherent accuracy of particular sensor types is important, but is only part-way down the path to optimization. Knowledge of sensor placement, the effects of environmental factors on sensor error, and calibration techniques that improve precision, that ultimately leads to optimum sensor selection.
Location and transient errors
It is nearly impossible to sense temperature exactly where you need it. The sensor itself has a finite size that displaces the sensing element from the ideal position for accurate measurement. Thermistors and RTDs are at greater risk for location error than an equivalently placed thermocouple — simply because of their size. In general, where a measurement must be pin-pointed, thermocouples are superior to both RTDs and thermistors (Figure 2). Thermocouple wire as small as 0.04 in. in diameter is available. Location error ‘A’ in the figure is a direct result of the entire sensor being displaced from its desired location; orientation is also a factor, especially at high temperature where the majority of heat transfer is by radiation. If surrounding heat sources and sinks are known, compensation can reduce the location error. However, this can be difficult in many systems because the understanding of the effect of sinks and sources may be unclear. When in doubt — use the simplest solution. Avoid complex calibration techniques and install the smallest sensor available as close to the temperature source as possible.
Transient errors are dynamic thermal errors. Typically, it is difficult to compensate for these errors — every material within the thermal system has its own unique thermal conductivity and capacity. For example, the thermoplastic covering thermocouple wire expands differently than the wire, which affects the resistance of the wire — aging affects the wire and the covering. Of the three most popular sensor types, it is the thermocouple that best minimizes transient errors because it is the smallest sensor with the smallest time constant.
Heat transfer error
Sensors receive conductive, convective and radiative inputs that contribute to measurement inaccuracy. This error can occur along a specific pathway, as when an electric wire of a sensor is heated by a nearby heat source. The measurement is thereby distorted. Heat transfer error affects thermistors, RTDs, and thermocouples. E and J thermocouples use alloys that are less conductive, which makes them ideal for mitigating this kind of error. When electrical resistance produces heat inside a sensor it causes a false-high reading — this is called “self-heating error.” This error applies to thermistors and RTDs only. The self-heating error is insignificant in flowing streams but can be a serious in static measurement. Strategies for minimizing this include keeping the current low or pulsing the sensor with a low duty cycle to keep the average power dissipated in the sensor low.
Atmospheric and environmental influences
All sensors are affected by aging, especially those in cyclic service, or when they are operated near their temperature limits. Material deterioration causes a drift from the initial profile (e.g., resistance increases). Thermocouples exhibit more complex behavior because the voltages produced are a direct result of the resistance difference between two dissimilar metals. Thermistors and RTDs are usually well-sealed from the environment making them less susceptible to internal corrosion. Thermistors usually exhibit some initial drift, but are generally stable after initial aging. Although sensors may be sealed, lead wire deterioration or corrosion is a problem regardless of sensor selection.
RTDs have a distinct advantage over other sensors. For RTDs, lead-wire corrosion problem is mitigated by using 3-wire or 4-wire units that effectively measure the resistance of the sensing element, versus the connection wire. With this type of installation, the RTD has the greatest overall stability of the three sensor types.
The effect of moisture on sensors is complex. For example, forced air flow on and around a sensor measuring a surface temperature will lead to heat transfer error. Convective currents add or remove heat from the sensor and measurement surface. If the atmosphere is at a different temperature than the surface, or the measurement environment is moist, the heat flow associated with convection must be considered as if it were another heat source or sink.
Mechanics and related effects
Small gage wire and fragile sensors should be avoided in applications that subject them to extreme mechanical motion, vibration or high-intensity acoustics. The most common wire failures occur near connection points, where there is the greatest amount of flexure. For example, a thermocouple wire is most vulnerable where it is bare wire in a process or in a thermowell. In addition, mechanical motion or vibration can stimulate internal resonances inside the sensor — leading to early failure. Thermocouples are generally the most durable of the three sensor types because many of the alloys used in the wires are more ductile — allowing them to handle additional motion. However, cold-working can increase the resistance of thermocouple wire, especially, in small wire.